Abstract

To better understand the factors determining Trichodesmium clade (Clade III vs. Clade I) distribution, a quantitative polymerase chain reaction method was developed and applied to a sample set from the western North Atlantic. Clade distributions were compared with a suite of physicochemical parameters on a cruise transect from Woods Hole, Massachusetts, to Barbados during the spring of 2011. Clade I comprised > 90% of the total Trichodesmium spp. population. Depth distributions of both clades were similar in waters where there was a shallow (< 20 m) mixed‐layer depth (MLD), but segregated in the water column where the MLD was deep (> 80 m). Surface abundances of both clades were higher when the MLD was < 20 m. Temperature and salinity were correlated with both clades in the data set as a whole. However, when stations influenced by a freshwater salinity anomaly were removed, dissolved inorganic phosphorus and total dissolved phosphorus correlated with Clade I cell density only. These correlations underscore the influence of phosphorus in driving Trichodesmium sp. dynamics in this region and are suggestive of niche partitioning between the two clades.

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